Lithium-ionic-conductive inorganic solid electrolytes having phosphorous as a component are abundantly known; for example the characteristics of an amorphous solid electrolyte whose composition is Li2S—P2S5 are set forth in Japanese Examined Pat. App. Pub. No. H05-48582 and in John H. Kennedy, et 2 al., “IONICALLY CONDUCTIVE SULFIDE-BASED LITHIUM GLASSES,” Journal of Non-Crystalline Solids 123 (1990), pp. 328-338.
Furthermore, in lithium-ion conducting, amorphous inorganic solid electrolytes, improving the lithium-ionic conductivity by the addition of an oxide to an inorganic solid electrolyte whose chief component is a sulfide has, as will be set forth below, been disclosed. Ways of manufacturing these inorganic solid electrolytes are by a rapid-quenching technique of a melt, or by a mechanical milling technique using a ball mill apparatus, in which raw materials react with each other. The form that the electrolytes take is chiefly powdery as well as discoid which the powder is molded into, or else quenched blocks or flakes.
A sulfide-based, highly lithium-ionic-conductive solid electrolyte that is added with an oxide is disclosed in Japanese Unexamined Pat. App. Pub. No. H04-202024. In the patent claims in this document, Li2O and LiOH as oxygen-incorporating lithium compounds to which Li2S—P2S5 sulfide is added are set forth, yet what the addition amount is for the oxygen-incorporating lithium compounds is not stipulated.
In Embodiment 3 of Japanese Pat. Pub. No. 3,343,936, the adding of Li3PO4 to Li2S—P2S5 sulfide is disclosed. In this embodiment it is shown that adding Li3PO4 at 3 mol % (2.7 atomic % oxygen content) improves the level of ionic conductivity and the breakdown-voltage quality of the electrolyte.
In the patent claims of Japanese Unexamined Pat. App. Pub. No. 2001-250580, an amorphous lithium-ionic-conductive solid electrolyte whose composition is aLi3PO4-bLi2S-cP2S5 is disclosed, with the ranges of the atomic fractions in the composition stipulated to be: a<0.3; b>0.3; c>0.2.
In Yoneda (with two others), “Mixed anion effect on lithium ion conductivity of amorphous materials in the system Li2O—Li2S—P2S5,” Extended Abstracts of Presentations at the 28th Symposium on Solid-State Ionics, (November, 2002), pp. 24-25, an Li2O—Li2S—P2S5 composition is set forth, wherein it is given that the level of ionic conductivity becomes highest when the oxygen content is 1.9 atomic %.
On the other hand, there are no reports of the incorporation of oxygen into crystalline solid electrolytes whose chief component is phosphorous: For example, in Pat. App. Pub. No. 2001-250580, and in Hama, et 4 al., “Synthesis and High Lithium Ion Conductivity of New Glass-ceramics in the System Li2S—P2S5,” Extended Abstracts of Presentations at the 26th Symposium on Solid-State Ionics, (November, 2000), pp. 174-175, heating a non-crystalline powder Li2S—P2S5 composition to crystallize it is set forth; and in Murayama, et 4 al., “Synthesis, Property and Structure of New Lithium Ionic Conductor, Thio-LISICON; Lithium Thiophosphate System,” Extended Abstracts of Presentations at the 28th Symposium on Solid-State Ionics, pp. 178-179, a crystalline solid-electrolyte composition, Li3+5xP1−xS4 (0≦x<0.3), in pelletized powder form is set forth.
Meanwhile, utilizing metallic lithium in negative electrodes as a technique for realizing heightened capacity in lithium secondary batteries has been attempted, but due during charging/discharging to reaction of the metallic lithium with the organic electrolytic solution contained within the battery, growth of dendrites of the lithium occurs on the negative electrode, giving rise to battery-internal shorting with the positive electrode, which presents a hazard that ultimately ends in the battery exploding. This dendritic growth is, moreover, thought to be a cause of degradation in charge/discharge capacity.
As one technique for curbing dendritic growth, on the surface of the metallic lithium either forming a polymer film, or forming an inorganic film such as a fluoride film, a carbonic film, an oxide film (claim 1 of the specification for U.S. Pat. No. 5,314,765), or a sulfide film (claim 4 of the specification for U.S. Pat. No. 6,025,094; claim 7 of Japanese Unexamined Pat. App. Pub. No. 2000-340257; claims 1-3 and 9 of Japanese Unexamined Pat. App. Pub. No. 2002-329524) have been disclosed.
In particular, in Pat. App. Pub. No. 2000-340257 and Pat. App. Pub. No. 2002-329524 oxygen being incorporated into a sulfide solid electrolyte is disclosed.